With no symptoms or details to go on, the first suspect is fuel pressure bleeding off overnight. That can be caused by a leaking fuel pump check valve or leaking fuel pressure regulator, neither of which is serious, or a leaking fuel injector. A leaking injector can allow a little raw fuel to run into the engine and wash the oil off the cylinder wall but any accelerated wear is going to be negligible. The biggest symptom will be long crank time before the engine starts. If low fuel pressure is the cause of a long crank time, you may be able to verify that by turning the ignition switch to "run", wait a few seconds, turn it off, wait a few seconds, then try to start the engine. Each time you turn the ignition switch on, the fuel pump will run for a second or two to build up pressure.